The invention relates to a process for the isomerization of a light paraffin feed stream comprising pentanes and hexanes to produce more highly branched paraffins of higher octane number and greater utility as naphtha boiling range motor fuel. The invention specifically relates to an improvement in the heat integration of the fractional distillation scheme used to recover a recycle stream and a product stream from the stabilized effluent stream of an the paraffin isomerization zone.
The majority of the naphtha boiling range hydrocarbons recovered from petroleum do not have the high octane numbers desired for modern gasolines. For instance, straight chain or relatively straight chain C5 and C6 hydrocarbons have octane numbers which are much lower than desired for gasoline blending components. As a result it is necessary for modern petroleum refineries to build high octane molecules, as by alkylation, and to increase the octane of existing straight chain molecules by isomerization. By isomerizing these straight chain molecules to more highly branched molecules the octane number of the molecules is increased.
Isomerization of naphtha boiling range hydrocarbons is affected by contacting the hydrocarbons with an isomerization catalyst at isomerization conditions. Unfortunately, such isomerization steps do not result in a complete conversion of the straight chain feed molecules, and a sizable percentage of the isomerate or product of this contacting consists of molecules which have only a moderate increase in branching. In order to further increase the octane number of the isomerization product the higher octane molecules are separated into a product stream while the relatively less branched, and therefore lower octane hydrocarbons, are concentrated into a recycle stream returned to the isomerization zone. Although adsorption is more effective in performing a division between the close boiling low and high octane molecules, it is also more costly. Therefore, this separation step is often performed by fractional distillation. The subject invention relates to fractional distillation steps performed in the recovery of high octane hydrocarbons and the recycling of low octane hydrocarbons to the isomerization zone.
The isomerization of light paraffins is a well known process performed in many petroleum refineries. As used herein the term xe2x80x9clight paraffinsxe2x80x9d is intended to refer to paraffins having five or six carbon atoms per molecule. An overall description of light paraffin isomerization technology is provided in Chapters 9.3 and 9.4 of the Handbook of Petroleum Refining Processes, second edition, Robert A. Meyers, editor; published in 1996 by McGraw-Hill. These chapters describe processing conditions, catalysts and process flows used in this process. Figure 9.3 illustrates the passage of the reactor effluent into a stabilizer column to recover a product labeled as isomerate. Figure 9.3.2 describes an optional flow in which the effluent of an isomerization reaction zone is passed into a deisohexanizer column which divides the entering material into an isomerate and a recycle stream, which is returned to the isomerization zone. Similar flow schemes with slightly more detail are shown in the paper entitled xe2x80x9cCatalyst and Engineering Innovations Improve Isomerization Economicsxe2x80x9d by R. J. Schmidt, et al., presented at the 1987 National Petroleum Refiners Associated annual meeting on Mar. 29-31, 1987, and in U.S. Pat. No. 4,804,803 issued to R. J. Schmidt et al.
The invention is an improved configuration of the fractional distillation zone used downstream of a light paraffin isomerization zone. The invention reduces the utility costs of this fractional distillation zone by reducing both the reboiler duty and the amount of cooling required to generate reflux liquid for the top of the column.
A broad embodiment of the invention may be characterized as a process for the isomerization of C5-C6 paraffins and the recovery of high octane, di-branched paraffins by fractional distillation, which process comprises passing a feed stream comprising C5-C6 paraffins into a catalytic reaction zone in which the feed stream is contacted with a paraffin isomerization catalyst maintained at isomerization conditions to yield an isomerization zone effluent stream comprising C5-C7 paraffins; passing the isomerization zone effluent stream into a first fractional distillation column maintained at fractionation conditions effective to remove as an overhead product substantially all hydrocarbons present in the isomerization zone effluent stream having less than 5 carbon atoms per molecule and forming a first net bottoms stream comprising C5-C7 paraffins; cooling the first net bottoms stream by indirect heat exchange against a first process stream; passing the first net bottoms stream into a second fractionation column at an intermediate elevation, with the second fractionation column operated at fractionation conditions effective to separate entering hydrocarbons into a net overhead stream, which is rich in relatively high octane C5 and C6 hydrocarbons including dimethyl butane produced in the isomerization zone, a sidecut stream removed at a lower intermediate elevation of the fractionation column and comprising C5 and C6 normal paraffins, and a net second bottoms stream comprising C7 hydrocarbons; withdrawing a first portion of the sidecut stream; employing a second portion of the sidecut stream as the first process stream referred to above, and, passing the second portion of the sidecut stream into the lower half of the second fractional distillation column at a third intermediate point.